Electrolytic oxidation of quinoline to quinolinic acid



Patented June 20, 1950 j f' ELECTROLYTIC OXIDATION of QUINOLINE 'ro QUmoLIn cAcm John B.;Conn, Westfield, N. J assignor to Merci:

& 00., Inc., Rahway, Jersey 1 'No Drawing.

I :1 V Y i I 1 This invention relates, to processes for the electrolytic oxidation of pyridine derivatives to their corresponding carboxy acids; and the present application is a continuation-in-part of my pending application Serial No. 418,013, filed November 6,1941, now abandoned. 1 The oxidationr-reduction effect of electrolysis is well known, but hitherto there has been relatively-little application of the principles of electrolysis to pyridine derivatives. Thei only jreported application of electrolytic oxidation to pyridine derivatives is that of Yokoyam-a (Bull. Chem. Soc, Japan, 7, 69-72) who oxidized a-picoline in dilutesuliuric acid medium at: a lead dioxide anode, using uniform current density of 5.0 amperes persquare decimeter. Yokoya-ma reports that the product he obtained contained only about 20-30% picolinic acid together with a number of by-products comprising ammonia, carbon dioxide, and formic acid. He also showed that picolinic acid is easily destroyed at a lead dioxide anode. The large proportion of by-products obtained by Yokoyama indicates the ease with which the pyridyl ring is split. .I have .now discovered an improved process for the electrolytic oxidation of pyridine derivatives. My new process has the advantages that splitting of the pyridyl ring is reduced to a minimum, and high yields of'the desired product are obtained.

According to my invention, the selected pyridine derivative is oxidized in about 6 to 12 N sulfuric acid medium at a lead anode coatedwith lead dioxide, and at low current density, pref erably under 2 amperes per square decimeter of anode surface. I have found a current density range of from to 1 /2 amperes per square decimeter of anode surface most satisfactory. During the later portion of the run, the current density is reduced to prevent the destruction at the anode of the pyridine carboxylic acid which has been formed. In some cases, for instance when quinoline is being electrolyzed, it is advisable to use a higher current density at the start of the electrolysis; otherwise there is a tendency for the lead dioxide anode to be reduced during the early part of the electrolysis. It is preferable to start the electrolytic oxidation of the pyridine derivation in about 6 N sulfuric acid medium. During the electrolysis, the concentration of sulfuric acid gradually increases, reaching about 12 N at the end of the oxidation process.

The cell used is of the two-compartment type, an acid resistant diaphragm, which may be of ,carborundum, an acid-resisting silicate, a glass N. 1., a corporation of New Application January '5, 1946, Serial No. 639,425. In CanadaMarch 20, 1942 2 Claims. (01., was);

2 I cloth, or other material having similar properties, being used to separate the anode compartment from the cathode compartment. The cathode compartment'is packed with a porous acidresisting substance, such as sand, in order to reduce its free volume to a fraction of that of the anode compartment. This compartment is kept filled with 1:1 (by volume) sulfuric acid (i. e. about 18 N acid).

The anode used in this cell is of lead coated with lead dioxide'by electrolysis for about 2 to 6 hours in about 6 N sulfuric acid; and is about 2.4 square decimeters insurface' area per mole of pyridine derivative to be oxidized, and may be of any convenient shape such as a cylindrical sheet, a coil, or a series of plates. This'anode is activated before use either'by continuous operation for 24 hours in about 6'N'sulfuric-acid in the presence of a depolarizer such as quinoline, pic'oline'or the like, or by reversing the polarity for live ten-minute intervals when operating at 1' ampere persquare decimeter in the absenceoi such a depolarizer. The anode can also be activated by actual operation. in the oxidation process cf th'e present invention, althoughwhen thus activated the current efl'iciency during the initial stages ofthe oxidation is very low. Whenthe anode'has been activated'by any of the methods described above, it remains'activate'd and can be used for a number of successive oxidations without'further activation. l i The cathode may be of any acid-resisting metal such as copper, a-highly resistant steel oran alloy (knownas chromel) oith e composition nickel 61%, iron 23%, chromium 16%. A nonmetal, such as graphite, may also be used as a cathode.

During the course of electrolysis, the temperature may be maintained at the desired level by any one of a number Of different convenient means such as by means of heating or cooling coils placed directly in the cell, or by pumping the anolyte through a heat exchanger. Stirring of the anolyte is advisable, and this may be accomplished by any convenient mechanical device, or, if desired, by an air jet placed directly in the anolyte.

The following examples illustrate methods of carrying out the present invention, but it is to be understood that these examples are given by way of illustration and not of limitation.

Example I The anode compartment of the two compartment cell is charged with a mixture of 96 cc. (1

face until 210 ampere hours per mole of ,B-picoline have been consumed. The current isthen reduced to 0.5 ampere per square decimeter of anode surface, and this lower current density is maintained for an additional 30 ampere hours. Toward the end of the run, the anolyte' becomes thick with suspended crystals of nicotinic acid hydrogen sulfate. The anode compartment. is

then emptied, washed out, and. refilled. with.

another charge of fl-picoline, acid andv water, a

and the anode is immediately returned to service to prevent inactivation. The anolyte liquor and washings are'combined, adjusted to pH 3 with sodium carbonate or sodium hydroxide, and allowed to stand until 0001. The nicotlnic acid which separates is filtered off. The yield is' 90 gms. per mol of 'p-picoline used. Additional acid may be recovered from the liquor by known methods. As for example; through the copper salt, the totalyie'ld is about 88%.

Example II The anode compartment of thetwo compartmnt cell is charged with a mixture of 50 gms. quin'oline and 200 gms. of concentrated (93%) sulfuric acid, diluted to 400- cc; with water; The current density is set at 4 to 5 amperes per square decimeter of anode surface until 100 ampere hours p'ermol of'quinoiinehave been consumed. The current density is then reduced'to 1.5 amperes per square'decimeter" of anode surface for anadditional 2.70 ampere hours permol of quinoline, and is finally reduced to 0.75 ampere per square decimeter of anode surface for a further 220 ampere hours'per moi. of quinolinei The. temperature during the run is maintained at 65-'-7'-5 C. by means of a heat exchange coil placed directly in? the anolyte. The-color of: the :anolyte, which is originally intensely red, fades during the run to pale orange. The cell i emptied, washed out, and recharged for another run.

The anolyte and anode washings are combined, diluted to 800 cc. with distilled water and adjusted to pH 3 with sodium hydroxide. The quinolinic acid is precipitated as the copper or ferrous salt by the addition of a slight excess of 4 copper sulfate or ferrous sulfate, and the free quinolinic acid is recovered by known methods. The yield is 117 gms. per mol of quinoline used.

Modifications may be made in carrying out the present invention without departing from the spirit and scope thereofiand I am to be limited only by the appended claims.

I claim:

1. The process for preparing quinolinic acid that comprises charging the anode compartment of a two compartment electrolytic cell, having a leaddioxide coated lead anode, with a mixture of qu-inoline in about 6 N sulfuric acid, agitating the anolyte and-oxidizing the quinoline therein by electrolysis at a current density of 4 to 5 amperes per square decimeter of anode surface for about IOOampere. hours per mole of quinoline, then at a current density of about 1.5 amperes per square decimeter for about 270 ampere hours per mole, and finally at a current density of about 0.75 ampere per square decimeter for about 220 ampere hours permole, andrecoveri'ng quinolinic acid thus formed from the electrolyte.

2. The process for preparing quinolinic' acid that comprises charging the anode compartment of a two compartment electrolytic cell, having a lead '-dioxide-coated lead anode, with a mixture of quinoline, about 93% sulfuric acid and water, in the approximate ratio of' gm. quinoline; 200 gm. acid, and water to bring the volume to 400 cc., agitating the anolyte and oxidizing the quinoline therein by electrolysis at a current density of 4 to 5 amperes per square decimeter of anode surface for about ampere hours per mole of quinoline, then at a current densit of about 1.5 am-peres per square decimeter for about 270 ampere hours per mole, and finally at a current density of about 0.75. ampere per square decimeter for about220' ampere hours per mole, and recovering' quinolinic acid thu formed from th 'elec trolyte.

J OHN B. CONN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Conn et a1 Nov; 16,1948

OTHER REFERENCES Number 

1. THE PROCESS FOR PREPARING QUINOLINIC ACID THAT COMPRISES CHARGING THE ANODE COMPARTMENT OF A TWO COMPARTMENT ELECTROLYTIC CELL, HAVING A LEAD DIOXIDE COATED LEAD ANODE, WITH A MIXTURE OF QUINOLINE IN ABOUT 6 N SULFURIC ACID, AGITATING THE ANOLYTE AND OXIDIZING THE QUINOLINE THEREIN BY ELECTROYSIS AT A CURRENT DENSITY OF 4 TO 5 AMPERES PER SQUARE DECIMETER OF ANODE SURFACE FOR ABOUT 100 AMPERE HOURS PER MOLE OF QUINOLINE, THEN AT A CURRENT DENSITY OF ABOUT 1.5 AMPERES PER SQUARE DECIMETER FOR ABOUT 270 AMPERE HOURS PER MOLE, AND FINALLY AT A CURRENT DENSITY OF ABOUT 0.75 AMPERE PER SQUARE DECIMETER FOR ABOUT 220 AMPERE HOURS PER MOLE, AND RECOVERING QUINOLINIC ACID THUS FORMED FROM THE ELECTROLYTE. 